Choke Assembly

ABSTRACT

A choke assembly comprises an inlet ( 48 ) for a multiphase fluid stream, the stream comprising a first relatively heavy fluid phase and a second relatively light fluid phase; a first fluid outlet ( 116 ); a choke element ( 22 ) disposed between the inlet and the first fluid outlet operable to control the flow of fluid between the inlet and the first fluid outlet; a separation chamber ( 40, 114 ) disposed to provide separation of phases in the fluid stream upstream of the choke element; and a second outlet ( 118 ) for removing fluid from the separation cavity. The choke assembly is of particular use in the control of fluid streams produced from a subterranean well, in particular oil and gas produced from a subsea well.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND

The present invention relates to a choke assembly for controlling theflowrate or pressure of a fluid. The invention relates in particular toa choke assembly for use in the control of flow of oil and/or gasstreams, especially in the control of fluid streams produced fromsubterranean wells. The choke assembly of the present invention isparticularly suitable for use in subsea installations for the drillingof subterranean wells and/or the subsea production of oil and gas.

The flowrate and/or pressure of a fluid stream are generally controlledin use by some form of valve assembly, in which the size of the orificeor conduit through which the fluid is caused to pass is altered. Aparticular form of assembly commonly employed for the control of fluidflowrate and/or pressure is a choke assembly. Known choke assembliescomprise a conduit having a plurality of orifices and a means forprogressively opening and closing some or all of the orifices to allowthe passage of fluid therethrough. The desired pressure and/or flowrateis determined by the number and size of orifices that are open andavailable for the passage of fluid.

A common design of choke assembly is of the plug and cage variety. Agenerally cylindrical cage is disposed within the choke body, the cagebeing provided with a plurality of openings or orifices therethrough. Acylindrical plug is disposed to be moveable co-axially with respect tothe cage, so as to open or close the orifices in the cage, dependingupon the position of the plug. The orifices in the cage are disposedalong the path of movement of the plug, such that movement of the plugfrom the fully closed to the fully open position along the longitudinalaxis of the cage opens successive orifices, thereby increasing thecross-sectional area available for fluid to flow. Typically, the fluidto be controlled is introduced from the inlet of the choke assemblyperpendicular to an annulus surrounding the exterior of the plug andcage, passes through the orifices of the cage into the interior of thecage and, from there to the outlet of the choke assembly. The orificesin the cage are disposed perpendicular to the longitudinal axis of thecage. Typically, equal and opposite orifices are used, to generate jetsof fluid entering the interior of the cage to impinge on one another,thereby dissipating the energy in the fluid stream.

In general, known choke assemblies of the aforementioned type have oneof two arrangements. In a first arrangement, the cage is fixed and theplug is moveable longitudinally with respect to the cage. The plugextends and is moveable within the cage. In a second arrangement, thecage is again fixed and the plug is moveable, but with the plug disposedexternally to the cage (generally known as a sleeve). In general, thearrangement employing an external cage and an internal plug ispreferred, as this provides a better degree of control of the fluidflowrate and/or pressure. However, there are several significantdrawbacks with both designs of choke, in particular the design employingan external cage.

It is the case that a subterranean well produces a fluid stream havingseveral phases of fluids. Liquid phases present in the fluid stream aretypically oil and water. Water is being produced from subterranean wellsin increasing quantities, for example as a result of operations toenhance oil recovery from a field by water injection. In addition, thefluid stream produced will typically contain significant volumes of gas.

Much effort is being put into developing systems to separate oil, waterand gas from the fluid stream produced by wells. In particular, giventhe increasing depths at which subsea wellhead installations areoperating, it is becoming increasingly desirable to avoid having toproduce water from the well to the surface. Rather, there is increasingneed to separate water produced from the well at the seabed, to allowfor reinjection.

However, conventional choke assemblies provide an obstacle to achievingthe desired fluid separation. The conventional plug/cage choke assemblyhas orifices extending perpendicularly through the cylindrical cage. Theorifices are typically circular. As a result of this arrangement, thefluid passing through the choke assembly is subjected to very high ratesof shear. This in turn generates significant mixing of the fluid phases,in some cases resulting in emulsification of the oil and water phases.This mixing significantly hinders the separation of the water, oil andgas phases.

Accordingly, there is a need for a choke assembly that provides therequired level of fluid control, without subjecting the fluid stream toexcessively high rates of shear.

U.S. Pat. No. 6,730,236 relates to a method for separating liquids in aseparation system having a flow coalescing apparatus and a separationapparatus. The separation system includes a flow conditioning apparatushaving an inlet and an outlet. A swirl chamber is disposed between theinlet and outlet and operates to create a swirling fluid flow pattern.It is suggested that this swirling pattern induces coalescence of liquiddroplets in the fluid stream. The flow conditioning apparatus comprisesan outer shell in which the fluid inlet is formed. The apparatus furthercomprises an inner swirl chamber having a helical pattern of tangentialholes, whereby fluid enters the swirl chamber from the outer shell andis caused to flow in a helical pattern. The flow of fluid into the innerswirl chamber is controlled by a plunger assembly, including a conicalhead that is moveable longitudinally within the swirl chamber.Reciprocation of the conical head covers and uncovers holes in thehelical pattern and allows the fluid flow to be controlled.

The flow conditioning system of U.S. Pat. No. 6,730,236 is intended foruse in conjunction with a downstream apparatus for separating the fluidphases and acts to condition the flow by inducing coalescence of fluidbubbles and, if required, to act as a choke device to control the fluidflowrate. It would be advantageous of a system could be provided thatcombines the operation of a choke and a separation action. In addition,it would be particularly useful if such a system could act to separatesolids from a fluid stream.

Further, the production of oil, water, and/or gas from a subterraneanwell, it is very often the case that the fluid stream has entrainedtherein significant quantities of solid material. The solid material,such as sand, silt and gravel, may be produced from the subterraneanformations along with the oil and gas. Sand and gravel entrained withthe oil and/or gas will enter the choke assembly together with the fluidstream. In addition, the well may produce quantities of metal particlesthat enter the choke, for example as a result of equipment wear orfailure upstream of the choke assembly.

In the designs of choke discussed hereinbefore, the cage assembly isparticularly vulnerable to damage from solids entrained with the fluidstream. Small particles entrained with the fluid generates a very highrate of wear on the cage and especially the plug, leading to poorcontrol of the fluid flowrate and/or pressure and eventual failure ofthe choke. In addition, with the arrangement of perpendicularlyextending, opposed orifices in the cage, solid particles and objectsentrained with the fluid are directed onto the portion of the interiorsurface of the cage opposite the orifice. In extreme, but not uncommoncases, large solid particles impacting the cage assembly can destroy thecage. In all such cases, the inevitable result is that the chokeassembly requires replacement. In the case of a subsea installation,perhaps at a depth of many thousands of feet of water, the replacementof a choke is a difficult, dangerous and time consuming task, duringwhich production from the well may need to be shutdown. Chokes havingthe external cage/internal plug design are particularly vulnerable todamage and failure from entrained solids. Choke assemblies located closeto the wellhead are particularly vulnerable to solids produced from thewell. However, as such choke assemblies are typically operating at orclose to wellhead pressure, their failure can lead to a potentiallydangerous situation and their replacement is a particularly difficulttask, especially at great depths of water.

Accordingly, there is a need for a choke assembly that reduces thedamage caused to the cage of the choke assembly by solids entrained withthe fluid being fed to the choke.

As noted, choke assemblies are required in order to reduce the pressureand/or flowrate of fluid streams, in particular fluid streams producedfrom subterranean wells. It has been found that this necessary chokeoperation can also be employed to provide a fluid separation functionand to condition the fluid stream for further downstream processing. Inparticular, it has been found that the choking operation can be employedto separate solid particles entrained or suspended in fluid streams.Developments have also been made to the choke assembly to prevent suchentrained and suspended solid particles from damaging the components ofthe choke, as hereinbefore described.

SUMMARY OF THE DISCLOSURE

According to a first aspect of the present invention, there is provideda choke assembly comprising:

an inlet for a multiphase fluid stream, the stream comprising a firstrelatively heavy fluid phase and a second relatively light fluid phase;

a first fluid outlet;

a choke element disposed between the inlet and the first fluid outletoperable to control the flow of fluid between the inlet and the firstfluid outlet;

a separation chamber disposed to provide separation of phases in thefluid stream upstream of the choke element; and

a second outlet for removing fluid from the separation cavity.

In the choke assembly, fluid enters through the inlet and flows towardsthe first or second outlet. Until separation of the fluid phases occursin the separation chamber, the entire fluid stream will flow in the samedirection. After separation, a portion of the fluid stream flows throughthe choke element to the first outlet, while the remaining portion ofthe fluid stream flows to the second outlet. References herein to‘upstream’ and ‘downstream’ will be used to indicate the relativepositions of features of the assembly and are to be construed in thelight of this overall flow scheme.

The choke assembly of the first aspect of the present inventioncomprises a choke element disposed between a fluid inlet and a firstfluid outlet. The choke element may be any arrangement that allows theflowrate and/or pressure of the fluid leaving through the first fluidoutlet to be controlled. One suitable choke element arrangement is theplug and cage arrangement discussed hereinbefore, that is a perforatedcage through which fluid may pass, with the flow of fluid through thecage being controlled by a moveable plug. Typically, the plug ismoveable longitudinally within the cage. The plug may be disposed eitherupstream, that is outside, or downstream, that is inside, the cage, asrequired. A preferred arrangement has the plug disposed within the cage.Preferred choke elements employing the plug and cage arrangement aredescribed hereinafter.

The choke assembly comprises a separation chamber disposed such thatseparation of the multiphase fluid stream occurs upstream of the chokeelement, such that all fluid passing through the choke element is causedfirst to flow through the separation chamber. A particularly preferredarrangement is one in which the choke element is disposed within theseparation chamber, especially in which the choke element is arrangedconcentrically within the separation chamber and the separation chamberextends around the choke element.

The choke element is positioned in the assembly to receive a portion ofthe fluid after separation of the fluid phases has occurred in theseparation chamber. The choke element is positioned to provide aflowpath for the lighter fluid factions, while allowing the heavierfluid fractions to remain within the separation chamber pass to thesecond outlet. Preferably, the choke element is disposed to receivefluid from a region of the separation chamber that is between the inletand the second outlet. In this way, a portion of the fluid in theseparation chamber is caused to pass through the choke element to thefirst outlet, while the remaining portion of the fluid bypasses thechoke element and flows to the second outlet.

The inlet for the fluid is disposed so as to direct the fluid stream tobe processed into the separation chamber. The inlet is preferablypositioned to direct fluid into a region of the separation chamber thatis displaced from the choke element, more particularly in a region ofthe separation chamber that is upstream of the choke element. In thisway, separation of the fluid stream is allowed to occur prior to thefluid reaching and passing through the choke element.

The inlet may be of any suitable shape and orientation. In a preferredembodiment, the inlet is rectangular or square in cross-section. Thispreferred shape provides advantages with respect to the preferredarrangement and operation of the separation chamber using a rotationalflow pattern for the fluid, as will be described hereinafter. Therectangular inlet may have any suitable aspect ratio, that is the ratioof the longest side to the shortest side. Preferably, the aspect ratioof the inlet is in the range of from 1:1 to 1:3.

The inlet may be oriented to direct the fluid into the separationchamber in any suitable manner. For example, the inlet may be positionedto direct incoming fluid radially into the separation chamber.Preferably, however, the inlet is oriented to direct the incoming intothe separation at an angle to the radius of the separation, in order togenerate a rotational flow pattern of the fluid in the separationchamber. Most preferably, the inlet is arranged at a tangent to theseparation chamber, whereby incoming fluid is directed along a flowpatharound the perimeter of the separation chamber.

The inlet may be oriented to direct fluid into the separation chamberperpendicular to the longitudinal axis of the separation chamber.However, it is preferred that the inlet is angled to direct fluid intothe separation chamber at an angle to the normal to the longitudinalaxis, such that the incoming fluid is directed into the separationchamber in a downstream direction. When the inlet is also arranged todirect the incoming fluid at an angle to the perpendicular to thelongitudinal axis of the separation chamber, the fluid is caused to flowin a helical path in the downstream direction within the separationchamber. Most preferably, the inlet is angled such that the fluidrotating within the separation chamber is caused to avoid impacting theincoming fluid, that is pass the inlet in a downstream position withsufficient distance to prevent the incoming fluid from contacting fluidalready rotating within the separation chamber. This considerablyreduces impact and collisions between fluid streams and impacts of fluidwith surfaces within the separation chamber, in turn reducing orminimising fluid shear.

To assist with creating a helical or rotational flow pattern for fluidwithin the separation chamber, the separation chamber is preferablyprovided with a fluid guide surface that extends in a helical patternwithin the separation chamber. Such a guide surface may be referred toas a ‘ramp’. In this way, fluid entering the separation chamber iscaused to rotate through contact with the fluid guide surface. The fluidguide surface is preferably used in conjunction with an appropriatelyangled and oriented inlet, as hereinbefore described.

The choke assembly is adapted for the processing of a multiphase fluidstream. The multiphase fluid stream may comprise two or more liquidphases. Alternatively, the fluid stream may comprise a gas phase and oneor more liquid phases. The choke assembly is particularly advantageouswhen applied to the processing of fluid streams produced fromsubterranean wells, in particular fluid streams produced from oil and/orgas wells. A typical multiphase fluid stream produced from a wellcomprises oil, water, most likely in combination with gas. In addition,the multiphase fluid stream may comprise solids, in the form ofsuspended or entrained solid particles. As noted above, fluid streamsproduced from subterranean wells typically contain suspended andentrained solid particles, for example sand, grit and larger particles.The choke assembly of the present invention is particularly suitable forthe processing and separation of multiphase fluid streams containingsolid particles.

The choke assembly may comprise a single inlet for fluid to beprocessed. Alternatively, the choke assembly may comprise a plurality ofinlets for providing fluid to the separation chamber. In particular, theassembly may comprise a first inlet for lighter fluid, such as lightliquids, for example oil, and gas, and a second inlet for heavier fluid,such as oil and water. The plurality of fluid inlets are preferablydisplaced from one another in the longitudinal direction of theseparation chamber, with the preferred arrangement being one in whichthe first inlet for lighter fluid streams is upstream of the secondinlet for heavier fluid streams.

The or each fluid inlet may be located adjacent one end, that is theupstream end, of the separation chamber. Alternatively, the or eachfluid inlet may be displaced in a downstream direction from the upstreamend of the separation chamber. The region of the separation chamberupstream of the inlet can serve to collect light fluids. In this way,when in operation and processing a fluid stream with very light fluidphases, such as gas, the light fluid phases are caused to collect andconcentrate in the upstream collection region, for example forming a gascap.

In a preferred embodiment, the choke assembly comprises a fluid inletassembly having a curved flow path, most preferably a helical flow path,for fluid entering the separation chamber through the fluid inlet. Inthis way, the incoming fluid is caused to flow along a helical flowpathbefore reaching the separation chamber. Separation of the fluid phaseswill begin to occur prior to the fluid entering the separation chamber,enhancing the separation efficiency of the choke assembly. The curvedflow path may be provided by an appropriately curved pipe, for example ahelical pipe assembly, or by suitable baffles or other flow directingmeans within the pipe connected to the inlet. The inlet assembly ispreferably oriented with respect to the fluid inlet so that the fluidstream entering the separation chamber is properly oriented to directthe heavier fluid phases into the region of the separation chamber withthe higher concentration of heavier fluid and the lighter fluid phasesinto the region of the separation chamber with the higher concentrationof lighter fluid. In particular, when the fluid in the separationchamber is to flow in a circular or helical path, the inlet assembly isoriented to direct the heavier fluid phases to the radially outerregions of the separation chamber and the lighter fluid phases to theradially inner regions of the separation chamber.

The second outlet is disposed to allow removal of fluid that has notleft the separation chamber through the choke element. The separationchamber, either by virtue of its form and/or its operation, serves todirect the heavier fluid phases, and any solids present, away from thechoke element and towards the second outlet. The separation chamberpreferably provides further conditioning of the fluid flow andseparation prior to the fluid leaving the chamber through the secondoutlet.

The second outlet is preferably located in the separation chamberdownstream of the choke element. The second outlet operates to removethe heavier fluid phases that have not entered the choke element. Whenthe choke assembly is used to process fluid streams containing solids,solid material will also leave the separation chamber through the secondoutlet. In order to promote the separation of the lighter and heavierfluid phases and, if present, solid particles, it is preferred that theregion of the separation chamber upstream of the second outlet, butdownstream of the choke element, is formed to allow gravity-assistedseparation to occur. In particular, it is preferred that any solidparticles are allowed to settle towards the second outlet.

To assist with allowing gravity separation in the region of theseparation chamber upstream of the second outlet, the separation chamberis preferably formed to provide a fluid flowpath of reducedcross-sectional area in the region upstream of the second outlet. Thisreduced flow area may be provided by having the outer wall of theseparation chamber tapered inwards, either continuously or stepwise.Alternatively, or in addition thereto, the separation chamber may beprovided with means to reduce the flow area, such as a tapered orconical member disposed co-axially within the chamber. In order toenable further separation of the fluid phases to take place, inparticular to allow the lighter fluid phases, especially gas, toseparate from the heavier fluid phase flowing towards the second outlet,the means for reducing the flow area is preferably provided with one ormore conduits therethrough, to allow lighter fluid phases to flowupstream towards the choke element.

In addition, the separation chamber is preferably provided with means toreduce or prevent fluid rotation within the upstream region adjacent thesecond outlet. This is particularly preferred when the separationchamber is to operate with a rotational fluid separation regime, asdescribed hereinbefore.

The choke assembly may comprise further means for separating the lighterfluid phases that leave the separation chamber through the chokeelement. In particular, the choke assembly may comprise further meansfor separating and removing a gas phase and a light liquid phase.Suitable and preferred means for effecting this separation will bedescribed hereinafter.

In a second aspect, the present invention provides a method ofcontrolling and separating the flow of a multiphase fluid stream, themethod comprising:

introducing the multiphase fluid stream into a separation zone;

allowing separation of the fluid phases in the fluid stream to occur inthe separation zone;

causing a first portion of the fluid stream to flow through a chokeelement to a first outlet;

controlling the flow of fluid through the choke element by adjusting thechoke element; and

causing a second portion of the fluid stream to flow to a second outlet.

In the method of the present invention, the multiphase fluid stream maycomprise a combination of gas and liquid phases, and/or a plurality ofliquid phases. The method of the present invention is particularlysuitable for the processing of multiphase fluid streams containing solidmaterial, for example suspended or entrained solids.

The multiphase fluid stream undergoes separation in the separation zone,resulting in a separation of the fluid phases into relatively lighterfluid phases and relatively heavier fluid phases. Any suitableseparation regime may be employed to cause the fluid phases to separate.The fluid stream is preferably caused to rotate in the separation zone,most preferably flowing in a helical pattern, in order to promote theseparation of lighter and heavier fluid phases and minimise the amountof shear forces applied to the fluid. The fluid stream is preferablyintroduced into the separation zone at an angle, in order to promote therotation of fluid within the zone. Most preferably, the fluid isintroduced tangentially into the separation zone. In order to preventthe incoming fluid from impacting the fluid already rotating within theseparation zone, the incoming fluid is preferably directed at an angleto the downstream direction, such that fluid rotating within theseparation chamber passes the incoming fluid stream without significantimpact between the two.

To enhance the rotational fluid separation, the incoming fluid may bepreconditioned prior to entering the separation zone by flowing along acircular flow path, most preferably a helical flowpath, to initiate theseparation of lighter and heavier phases. If such preconditioningoccurs, the preconditioned fluid stream is preferably introduced intothe separation zone such that the fluid phases are respectively orientedaccording to the arrangement of fluid phases within the separation zone.In particular, the incoming fluid stream is preferably oriented with theheavier phases entering closest to the radially outer region of theseparation zone and the lighter phases being closest to the radiallyinner region of the separation zone.

In operation, the lighter fluid phases are caused to pass through thechoke element. The lighter fluid phases entering the choke element maycontain heavier fluid phases, but in significantly reduced amountcompared with the fluid stream at the inlet, that is the fluid enteringthe choke element will be richer in lighter fluid phases. In contrast,the heavier fluid phases and, if present, solid material, are caused tobypass the choke element and pass towards the second outlet for leavingthe separation zone.

The heavier fluid phases remaining in the separation zone and flowtowards the second outlet will comprise lighter fluid components, but berich in the heavier phases. Most preferably, the heavier fluid phasesundergo further separation in the separation zone, in order to furtherremove lighter fluid phases. This separation is most preferably agravity assisted separation, with any rotation of the fluid stream beingdamped or hindered by suitable means in the separation zone.

As noted above, the fluid stream is separated in the separation zoneinto respectively heavier and lighter phases. It is preferred that thelighter phases are caused to flow through the choke element to the firstoutlet, while the heavier phases remain in the separation zone and flowto the second outlet. Accordingly, in a further aspect, the presentinvention provides a method of controlling and separating the flow of amultiphase fluid stream, the method comprising:

introducing the multiphase fluid stream into a separation zone;

allowing separation of the fluid phases in the fluid stream to occur inthe separation zone;

causing lighter fluid phases separated in the separation zone to flowthrough a choke element to a first outlet;

controlling the flow of fluid phases through the choke element byadjusting the choke element; and

causing the remaining fluid phases to flow to a second outlet.

As described hereinbefore, the methods of the present invention may beoperated to provide a single fluid stream as feed to the separationzone. However, in one preferred arrangement, as described above, thechoke assembly is provided with a plurality of inlets, in particular afirst inlet for lighter fluid phases and a second inlet for heavierfluid phases. Accordingly, the present invention also provides a methodof controlling and separating the flow of multiphase fluid streams, themethod comprising:

introducing a first fluid stream rich in lighter fluid phases into aseparation zone;

introducing a second fluid stream rich in heavier fluid phases into aseparation zone to form a combined fluid stream in which the first fluidstream and the second fluid stream are in contact;

allowing separation of the fluid phases in the combined fluid stream tooccur in the separation zone;

causing lighter fluid phases separated in the separation zone to flowthrough a choke element to a first outlet;

controlling the flow of fluid phases through the choke element byadjusting the choke element; and

causing the remaining fluid phases to flow to a second outlet.

It is the case that light fluid streams, in particular gas streams,typically have minor portions of heavier fluids, in particular entrainedliquid droplets, which are to be removed. Equally, heavier fluidstreams, in particular liquid streams such as oil and water, will haveminor portions of lighter fluids, in particular entrained gas, which isalso to be removed. The method of the present invention allows thelighter and heavier streams to contact in the separation zone, but notintimately mix, and form an interface across which the minor componentsin each stream can pass. In this way, the heavier fluid components, inparticular liquid, leave the first, lighter fluid stream. Similarly, thelighter fluid components present in the second, heavier fluid streampass across the interface to collect in the lighter fluid stream.

As noted previously, it is preferred that the first fluid stream isintroduced into the separation zone upstream of the second fluid stream.This allows a stable fluid-fluid interface to be formed rapidly andmaintained. Both the first and second fluid streams are preferablycaused to rotate within the separation zone, as hereinbefore described,to enhance separation. Either one or both fluid streams may bepreconditioned to initiate separation prior to entry into the separationzone, again as hereinbefore described.

The choke element receives fluid that has been separated into lighterand heavier phases. In a preferred arrangement, the lightest fluidphases, in particular gas, enters the choke element in the most upstreamportion, with the heavier fluid phases, in particular light liquidphase, entering in the adjacent downstream portion and, if present,heaviest fluid phases entering the most downstream portion of the chokeelement. This flow scheme offers significant advantages in the operationof choke assemblies having further separation means downstream of thechoke element, as described below.

As described hereinbefore, it is advantageous to provide the chokeassembly with a plurality of fluid inlets, in particular a first inletfor a lighter fluid stream and a second inlet for a heavier fluidstream. Accordingly, the present invention also provides a chokeassembly comprising:

a choke element for controlling the flow of fluid therethrough;

a first inlet for a lighter fluid stream;

a second inlet for a heavier fluid stream; and

at least one outlet for fluid.

In addition, the present invention also provides a method forcontrolling the flow of fluid, the method comprising:

introducing a first, light fluid stream into a choke assembly through afirst inlet;

introducing a second, heavy fluid stream into the choke assembly througha second inlet;

allowing the first and second fluid streams to come into contact;

passing fluid from the first and second streams through a choke elementto control the flow of fluid; and

removing fluid from the choke assembly through at least one outlet.

As noted above, the choke assembly of the present invention may comprisemeans for separation of the fluid stream that has passed through thechoke element. Accordingly, in a further aspect, the present inventionprovides a choke assembly comprising:

an inlet for a multiphase fluid stream;

a choke element;

a separation chamber located downstream of the choke element to receivefluid passing through the choke element;

a first outlet in the separation chamber for a first fluid stream; and

a second outlet in the separation chamber for a second fluid stream.

This separation means for the fluid stream is preferably in addition tothe features of the choke assembly described hereinbefore, in particularthe separation features upstream of the choke element.

The choke element may have any suitable design and configuration toprovide the necessary control of the fluid stream passing therethroughto the outlets. A most suitable arrangement for the choke element is aplug and cage assembly, as discussed hereinbefore, preferred embodimentsof which are described hereinafter.

In the choke assembly of this aspect of the present invention, theseparation chamber may have any suitable configuration. It is preferredthat the separation chamber is arranged to allow separation to occur asa result of the rotational flow of the fluids therein, in particular byhaving the fluid stream flow in a helical pattern. Any suitable meansmay be provided to promote the rotational or helical fluid flow pattern.Preferably, the components of the choke element are adapted to induce arotational flow pattern in the separation chamber. Suitable andpreferred choke elements are described hereinafter.

In the choke assembly, a first outlet for fluid is provided. This outletis for lighter fluid phases, in particular gaseous phases. A secondoutlet is provided in the separation chamber for the remaining fluid, inparticular heavier fluid phases.

In one preferred embodiment, the separation chamber extends within thechoke element. This arrangement is particularly preferred when operatingthe assembly to control and separate fluid streams comprising asignificant gas phase. The separation chamber is preferably arranged sothat, in operation, gas collects in the upstream region of theseparation chamber and forms a gas cap. Gas collected in the gas capleaves the separation chamber through the first outlet, while liquidremaining in the separation chamber exits through the second outlet.

In a preferred arrangement, the choke element comprises a stem connectedto a choke actuator or drive mechanism. This arrangement is particularlypreferred when using a choke element of the plug and cage type, in whichthe plug is moved longitudinally within the cage by means of a stemconnecting the plug to an actuator or suitable drive means. The stem ofthe choke element is formed with a central bore, and provides the firstoutlet for fluid from the separation chamber, in particular for gas.

As heavier fluids can be entrained in the lighter fluid phase leavingthrough the first outlet, the choke assembly may be provided with afurther separation means for removing such entrained heavier fluids. Theseparation means preferably comprise an inlet for lighter fluid,connected to the first outlet from the separation chamber, and a secondseparation chamber having a first outlet for lighter fluid phases, inparticular gas, and a second outlet for heavier fluid phases. Mostpreferably, the separation means are arranged to provide, in operation,a rotational flow of fluids within the second separation chamber,whereby the heavier liquid phases are collected in and removed from theradially outer regions of the second separation chamber and the lighterfluid phases are collected in a removed from the radially inner regionsof the second separation chamber.

Fluids, in particular heavier fluid phases, remaining in the separationchamber flow to the second outlet, through which they exit. Preferably,the choke assembly comprises a means to inhibit or prevent the formationof a stable vortex in the fluids in the region of the separation chamberadjacent and upstream of the second outlet. Suitable means for arrestingor preventing vortices are known in the art and include a vortexbreaker. The need for a vortex breaker is particularly great when thefluids being separated in the separation chamber include gas, which isremoved through the first outlet, and the separation occurs usingrotational flow of the fluids.

The separation chamber may have any suitable configuration and shape. Ina preferred arrangement, the chamber is formed to have an increasingcross-sectional area to fluid flow in the region upstream and adjacentthe second outlet.

As noted, one preferred embodiment of the present invention provides ameans for removing a light fluid phase, in particular a gas, from thechoke assembly through the stem of the choke element. Accordingly, in afurther aspect, the present invention provides a choke assemblycomprising:

a choke element comprising a moveable choke component;

a stem connected to the moveable choke component;

wherein the stem has a bore therethrough to provide an outlet for fluidfrom within the choke element.

The separation means downstream of the choke element for fluid passingthrough the choke may comprise further separation means for separatingtwo liquid phases, either as an alternative to or in addition to theseparation described previously in which a lighter fluid phase, inparticular a gas is removed. The further separation means are preferablydisposed in the separation chamber downstream of the choke element andprovide an outlet for a first liquid phase and an outlet for a secondliquid phase. One preferred arrangement is employed in conjunction witha rotational separation regime, in which the fluids in the separationchamber are caused to rotate. As described, under the action of therotating fluids, the lighter fluids are caused to collect in theradially central region of the separation chamber. In such a case, anoutlet is preferably disposed in the radially central region of theseparation chamber, in order to remove the lighter liquid phase. Theoutlet is most preferably disposed at the longitudinal axis of theseparation chamber in the form of a longitudinal conduit extendingco-axially within the separation chamber. The conduit is preferablyprovided with a plurality of openings, through which the lighter liquidphase may pass to enter the conduit. It is preferred that the openingsextend at an angle to the radial direction of the conduit, mostpreferably tangentially, in order to provide a flowpath that subjectsthe liquids to minimum shear.

Heavier liquid phases collected at the raidally outermost regions of theseparation chamber are removed through an outlet disposed in theoutermost wall of the separation chamber. Again, this outlet is mostpreferably arranged at an angle to the radial direction of theseparation chamber, especially tangentially.

The present invention provides in a further aspect, a method forcontrolling and separating a multiphase fluid stream, the methodcomprising:

passing the fluid stream through a choke element and controlling theflow of fluid using the choke element;

introducing the fluid stream into a separation zone and causing phasesof the fluid stream to separate;

removing a lighter fluid phase from the separation zone through a firstoutlet; and

removing a heavier fluid phase from the separation zone through a secondoutlet.

Separation in the separation zone may take place using any suitableregime. However, it is preferred that the separation takes place using arotational fluid flow, in particular a helical fluid flow through theseparation zone. In this way, heavier fluid phases are caused to collectin the radially outermost regions of the separation zone, while lighterfluid phases collect in the inner radial regions. In a preferredembodiment, the separation zone extends within the choke element. Inparticular, the lighter fluid phases, especially gas, is allowed tocollect within the upstream region of the choke element, from where itis removed through the first outlet. A most convenient method removesthe lighter fluid phases from the separation zone within the chokeelement through the stem of the choke assembly. Accordingly, the presentinvention also provides a method for controlling and separating amultiphase fluid stream, the method comprising:

passing the fluid stream through a choke element and controlling theflow of fluid using the choke element;

introducing the fluid stream into a separation zone and causing phasesof the fluid stream to separate; and

removing a lighter fluid phase from the separation zone through anactuation stem extending from the choke element.

The method may further comprise separating heavier fluid phases, inparticular liquid phases within the separation zone downstream of thechoke element. Any suitable separation regime may be used to separatethe liquid phases. Most preferably, a rotational flow regime isemployed, in particular with the fluids flowing in a helical pattern,whereby the lighter liquid phases collect in the radially innermostregion of the separation zone and the heavier liquid phases collect inthe radially outermost regions. The lighter liquid phases are removedfrom the innermost region, most preferably through a conduit extendinglongitudinally within the separation zone. The heavier liquid phases arepreferably removed from the outer region of the separation zone,especially through an outlet arranged at an angle, preferablytangentially, to the separation zone.

As noted above, the present invention provides significant improvementsin the design and arrangement of choke elements, in particular chokeelements of the plug and cage type. In this variety of choke assembly,the cage is provided with a plurality of openings therethrough to allowthe passage of fluid. The plug is moveable with respect to the cage and,by overlying the openings, is used to open or close the openingsaccording to the flow control requirements of the choke. The plug ismoved with respect to the cage between an open position, in which all ofthe openings in the cage are uncovered and are open for the passage offluid therethrough, and a closed position in which all openings in thecage are covered and closed to fluid flow. Movement of the plug withrespect to the cage between the open and closed positions successivelyopens or closes the openings, depending upon the direction of movement.

A particularly preferred choke assembly is one in which the openings inthe cage extend at an angle to the radial direction of the cage, mostespecially at a tangent to the cage. In this way, the fluid passingthrough the cage is caused to enter a rotational flow regime within thecage and downstream thereof. The angled or tangential entry of fluidinto the choke cage reduces the impact of the individual fluid streamsentering through a plurality of openings and, hence, significantlyreduces the degree of shear to which the fluids are subjected. In thisway, any separation of fluid phases that has occurred or been initiatedupstream of the choke element is not affected. In addition, by causingthe fluids to follow a curved or tangential path within the choke cage,the direct impact of entrained and suspended solid particles enteringthe cage on the opposing inner wall portion of the cage are reduced oreliminated. In this way, damage to or destruction of the choke cage issignificantly reduced.

The rotational fluid flow pattern generated by the angled or tangentialflow of fluids within the cage is of particular use with the downstreamseparation of the fluids, as described hereinbefore.

In a first aspect relating to choke elements, the present inventionprovides a choke assembly comprising a choke element comprising:

a cage having a plurality of openings therethrough for the passage offluid;

a plug moveable with respect to the cage to open and close the openingsin the cage;

wherein the openings in the cage extend tangentially to the cage, eachopening comprising an outer portion extending from the outer surface ofthe cage having a first cross-sectional area and an inner portionextending from the inner surface of the cage having a secondcross-sectional area, the first cross-sectional area being greater thanthe second cross-sectional area.

The openings are shaped to decrease in cross-sectional area in theinwards direction. In this way, the fluid passing through the cage iscaused to pass along an increasingly smaller conduit, thereby increasingits velocity. This serves to assist with the formation of a rotationalfluid flow pattern within the choke element and downstream thereof,while at the same time reducing the degree of shear to which the fluidis subjected. The openings may have a step-wise change incross-sectional area. More preferably, in order to reduce the shear towhich the fluid is subjected, the changes in cross-sectional area aregradual or continuous.

The form of the openings, having an outer portion that is wider andproviding a greater cross-sectional area for fluid flow than the innerportion, is advantageous when used with a choke element that has two,concentric portions. This form of choke element has an inner, generallycylindrical choke portion of a hard, resistant material, such astungsten. A generally cylindrical outer portion extends coaxially aroundthe inner portion and may be formed of a less resistant material, suchas stainless steel. The form of opening allows the fluid to pass throughthe outer, less resistant portion at a lower velocity and only have ahigher, more erosive velocity when passing through the inner chokeportion.

As noted, the openings through the cage are shaped, such as beingtapered or curved, with the cross-sectional area of the opening reducingin the inwards direction. A tapered opening having substantiallystraight or linear sides is easier to form in the cage than a curvedopening. While more difficult to fabricate, openings with a curvedprofile may be preferred as they provide an optimum flow pattern offluid through the cage elements. The openings may each be symmetricalabout their central axis extending through the cage. However, in onepreferred arrangement, the openings are offset, such that the outer endof the opening, having the widest cross-sectional area, extends in adirection upstream of the flow of fluid past the choke cage when inoperation. This has the effect of reducing the shear experienced by thefluid as it enters the opening in the cage.

In one preferred arrangement, the choke cage is formed from an innerchoke cage element and an outer choke cage element, arrangedconcentrically around the inner choke cage element. The inner and outerchoke cage elements are formed with corresponding openings to providepassage for fluid through the cage. The outer choke cage element may beformed with openings having the first, greater cross-sectional area andthe inner choke cage element may be formed with openings having thesecond, lesser cross-sectional area. In one embodiment, the openings inthe outer cage element taper inwardly. The openings in the inner chokecage element may have a constant cross-sectional area along their lengththrough the element.

The ratio of the first and second cross-sectional areas is preferably inthe range of from 1:1.5 to 1:5, more preferably from 1:2 to 1:3. Theopenings are sized to provide a graduated, most preferably a smooth,gradual, entry for the fluid rotating around the outside of the cage tothe tangentially arranged opening and into the inner region of the cage.

In a further aspect of the present invention relating to choke elements,there is provided a choke assembly comprising a choke elementcomprising:

a cage having a plurality of openings therethrough for the passage offluid;

a plug moveable with respect to the cage to open and close the openingsin the cage;

wherein the openings in the cage extend tangentially to the cage, thecage comprising openings arranged in a plurality of bands extendingcircumferentially around the cage, the cross-sectional area of theopenings of the bands increasing in the upstream direction of the cage.

The arrangement of the openings into bands allows for an accuratecontrol of the flow of fluid through the choke, by positioning the plugwith respect to the cage accordingly. In operation, the most downstreamband of openings is the last to be covered when the choke is beingclosed and the first to be uncovered when the choke is being opened. Inthis way, as the choke is opened and each successive band of openings isuncovered, the cross-sectional area available for the flow of fluidincreases by an increasing amount with successive bands.

Each band may have a different cross-sectional area of openings thanthose adjacent to it. Alternatively, the bands may be grouped, such thatall the bands in a given group have the same cross-sectional openingarea, but have a greater area than the bands in the adjacent downstreamgroup. In a preferred embodiment, the bands are in groups of two, suchthat each pair of bands differs in cross-sectional area of openings toeach adjacent pair of bands.

The openings in a given band preferably have the same cross-sectionalarea. In one embodiment, all the openings in the bands have the samecross-sectional area and the difference in the cross-sectional area ofopenings between bands is achieved by varying the number of openings insuccessive bands or groups of bands. For example, in one preferredarrangement, each band in the most downstream pair of bands have twoopenings. The bands in the adjacent pair of bands each have fouropenings. The adjacent upstream pair of bands each have 8 openings.

It is preferred that the openings in adjacent bands are offsetcircumferentially with respect to one another, in particular whereadjacent bands are in the same group and have the same number ofopenings.

In the choke assemblies of the present invention, the plug and cage aremoveable with respect to one another. Preferably, the cage is fixed andthe plug is moveable with respect to the cage. In a very typicalarrangement, the plug is arranged concentrically with respect to thecage and moves longitudinally along the central axis shared by the plugand cage. The plug may be disposed outside or inside the cage, with theplug being disposed inside the cage preferred.

It has been found that the arrangement of openings in the cage intobands allows for a different choking regime to be achieved within thechoke cage than known choke assemblies. In the known arrangements ofchoke assemblies, the flow of fluid through the choke elements iscontrolled or choked by varying the cross-sectional area of the openingsin the choke element, in particular the cage, through which the fluidmust flow. This choking regime relies upon the physical barrierpresented by the choke cage to the flow of fluid. It has now been foundthat an alternative regime may also be employed. In particular, thepassage of fluid through bands of angled openings, especially tangentialopenings, in the choke cage establish for each band of openings a bandor ring of rotating fluid within cage. The shape adopted by the rotatingfluid in the cage is generally toroidal. Fluid entering through anopening in an upstream band must flow past the rotating fluid band orring established by a downstream band of openings. It has been foundthat the fluid is caused to move radially inwards to pass the downstreamband, that is the rotating band of fluid causes an effective reductionin the cross-sectional area of the interior of the cage available forfluid flow. The use of a plurality of bands of openings to generate acorresponding plurality of rotating bands of fluid can provide aneffective regime for choking and controlling the flow of fluid throughthe choke assembly. This use of the fluid itself to limit or control thecross-sectional area available for fluid flow provides an effectivechoking mechanism, while subjecting the fluid to very low rates ofshear.

The change in direction of flow of the fluid in this way is accompaniedby a change in momentum of the fluid stream, which in turn creates aresistance to the flow of fluid or a back pressure, allowing theflowrate of the fluid stream through the choke assembly to becontrolled.

Accordingly, in a further aspect, the present invention provides amethod of controlling the flow of a fluid, the method comprising:

introducing the fluid into a flow control zone having a generaldownstream direction in which the fluid is required to flow, the fluidbeing introduced into the flow control zone through a plurality ofopenings; wherein

fluid introduced through a first group of openings, downstream of asecond group of openings, establishes a generally toroidal flow patternwithin the flow control zone, whereby the effective cross-sectional areaavailable for the flow of fluid introduced through the second group ofopenings in the downstream direction is reduced.

The first group of openings may comprise a single opening, or morepreferably a plurality of openings, in order to establish the toroidalflow pattern within the flow control zone. The openings are such thatthe fluid enters at an angle to the radial direction of the flow controlzone, most preferably tangentially into the flow control zone.

The openings in the second group may be such that fluid introduced intothe flow control zone through these openings is caused to form a secondtoroidal flow pattern, providing a further reduction in the effectivecross-sectional area available for the flow of fluid introduced upstreamof the second group. Third and further groups of openings may beemployed to generate further toroidal flow patterns.

As noted previously, the methods and apparatus of the various aspects ofthe present invention are particularly suitable for use in theprocessing of fluid streams produced from subterranean wells, inparticular fluid streams produced from oil and gas wells. Accordingly,in a further aspect, the present invention provides an installation forthe processing of fluid streams produced from a subterranean wellcomprising an assembly as hereinbefore described. The efficientseparation of fluid streams produced from subterranean wells, inparticular the separation of gas, water and oil phases from a producedfluid stream, is particularly important in the case of subsea wells. Itis commonly the case that water produced from a well is required to beseparated from co-produced oil and gas and pumped back into thesubterranean formations. The choke assemblies of the present inventionmay be advantageously applied in the separation of water from a fluidstream produced from a subsea well, allowing the water to be returned tothe underground formations without the need for the fluid stream to beproduced all the way to the surface of the sea. Accordingly, the presentinvention also provides a subsea installation comprising a chokeassembly as hereinbefore described.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, having reference to the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a first embodiment of a chokeassembly of the present invention;

FIG. 2 a is a cross-sectional view of the upper portion of the chokeassembly of FIG. 1, with the inlet assembly shown in greater detail;

FIG. 2 b is a representation of the fluid flow pattern in a portion ofthe inlet assembly;

FIG. 3 is a cross-sectional representation of the fluid flow patternwithin the choke assembly of FIG. 1 along the line I-I;

FIG. 4 is a representation of the fluid flow pattern within theseparation chamber of the choke assembly of FIG. 1;

FIG. 5 is a cross-sectional view of a second embodiment of a chokeassembly of the present invention;

FIG. 6 is a cross-sectional view of a third embodiment of a chokeassembly of the present invention;

FIG. 7 is a cross-sectional view of a fourth embodiment of a chokeassembly of the present invention;

FIG. 8 is a representation of the fluid flow pattern within theseparation chamber of the choke assembly of FIG. 7;

FIG. 9 is a cross-sectional view of a fifth embodiment of a chokeassembly of the present invention;

FIG. 10 is a cross-sectional view of a sixth embodiment of a chokeassembly of the present invention;

FIG. 11 is a cross-sectional view of a choke element of the presentinvention;

FIG. 12 is a cross-sectional view of a portion of the choke cage of thechoke element of FIG. 11 showing a detail of the opening therethrough;

FIGS. 13 a to 13 f are cross-sectional views through the choke cage ofthe choke element of FIG. 11 along the lines Section A, Section B,Section C, Section D, Section E and Section F, respectively; and

FIGS. 14 a and 14 b are diagrammatic representations of fluid flowpatterns during the operation of the choke assembly of FIG. 11.

DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENTS

Referring to FIGS. 1 and 2 a, there is shown a choke assembly, generallyindicated as 2. The choke assembly comprises a generally cylindricalhousing 4 having an inlet end, generally indicated as 6, and an outletend, generally indicated as 8. A first cap 10 is mounted on the housing4 by bolts 12 to enclose the inlet end 6. A second cap 14 is mounted onthe housing 4 by bolts 16 to enclose the outlet end of the housing. Thefirst and second caps 14, 16 are formed to provide several additionalfunctions, as follows:

The first cap 10 has a central mandrel 18 extending therefrom coaxiallywithin the housing 4, the distal end of the mandrel 18 being formed witha cylindrical recess 20 to provide a housing and support for the upperportion (as viewed in FIG. 1) of a choke element, generally indicated as22. The choke element may be of any conventional design, with thearrangement shown in FIG. 1 being of a plug and cage type. The detailsof a preferred choke element are shown in FIG. 11 and describedhereinafter. An actuator 24 is mounted to the exterior of the first cap10 in conventional manner. The first cap 10 and mandrel 18 are providedwith a central longitudinal bore 26, extending from the actuator 24 tothe cylindrical recess 20, through which extends an actuator stem 28.The actuator stem 28 is connected to the plug of the choke element 22,again in conventional manner, and provides means for the actuator 24 tomove the plug longitudinally within the choke cage. The second cap 14has a central mandrel 30 extending therefrom coaxially within thehousing 4, the distal end of the mandrel 30 being formed to support thelower portion (as viewed in FIG. 1) of the choke element 22. The centralmandrel 30 and second cap 14 have a large bore 32 extendinglongitudinally therethrough. At its inner end, the bore 32 opens intothe central cavity within the choke element. At its outer end, the bore32 extends through an outlet nozzle 34 formed on the exterior of thesecond cap 14, terminating in a flange 36 of conventional design. Inthis way, the bore 32 provides a conduit for fluid passing through thechoke element 22 to leave the choke assembly 2.

As will be seen in FIG. 1, the mandrels 18, 30 and the choke element 22,together with the inner surface of the housing 4, define a generallyannular cavity which, in operation, serves as a separation chamber 40.An inlet nozzle 42 for fluid to be processed is provided in the inletend 6 of the housing 4, terminating in a flange 44 of conventionaldesign. The housing is provided with an outlet 46 for fluid extendingthrough the housing wall adjacent the second cap 14 at the outlet end 8.The details of the separation chamber 40, the inlet nozzle 42 and theoutlet 46 will now be described in more detail.

A fluid inlet assembly 48 is connected to the inlet nozzle 42 by way ofthe flange 44, the generally arrangement of which is shown in dottedlines in FIG. 1 and in detail in FIG. 2 a. The inlet assembly 48comprises a helically extending pipe 50, with the pipe being angled toallow solid material to move along the pipe towards the inlet aided bygravity. The angle of the pipe 50 will be determined by that required toensure movement of the solids content of the fluid stream beingprocessed. Typically, the pipe 50 will be at an angle of from 5 to 25°,more preferably about 10°. In operation, the inlet assembly 48 providesa pre-conditioning for the multiphase fluid stream being processed. Theflow of fluid through the helical path within the pipe 50 causesseparation of the heavier and lighter phases in the stream to begin,with the heavier phases collecting at the radially outer region of thepipe bore and the lighter phases moving towards the radially innerregion of the pipe bore. FIG. 2 b shows a representation detail of thefluid phase separation that is initiated in the bore of the pipe 50 ofthe inlet assembly 48.

The inlet assembly 48 is arranged such that fluid entering theseparation chamber 40 through the inlet nozzle 42 is correctly orientedwith respect to the fluid within the separation chamber. The orientationof the fluid entering the separation chamber 40 is shown in FIG. 3,which is a cross-sectional representation of the inlet end 6 of theseparation chamber 40 and the inlet nozzle 42. The incoming fluid isoriented to have the heavier phases introduced adjacent the inner wallof the housing 4 and the lighter phases introduced towards the centrallongitudinal axis of the housing 4.

The components at the inlet end 6 of the choke assembly 2 are arrangedto create a descending helical flow pattern for the fluid within theseparation chamber 40. As shown in FIG. 3, the inlet nozzle 42 isarranged to open tangentially into the separation chamber 40. The inletnozzle 42 has a rectangular bore 52 inclined at an angle to thelongitudinal axis of the housing 4 of about 85°, that is an angle of 5°from normal to the longitudinal axis of the housing. Other angles ofentry may be used, depending upon the nature of the fluid stream to beprocessed and the overall geometry of the separation chamber 40. Theangle of entry may, for example, range from 0 to 25°. This angled entryensures that fluid entering the separation chamber is caused to follow ahelical path downstream within the separation chamber 40. The helicalflow pattern of the fluid entering the separation chamber is representedin FIG. 4. The angle of entry is selected to ensure that the incomingfluid is not brought into direct contact with the rotating fluid alreadyin the separation chamber, but rather that the entering fluid, uponcompleting one revolution within the separation chamber 40 is caused topass downstream of the inlet bore 52. To further assist withestablishing the helical fluid flow pattern and to reduce contactbetween the incoming fluid and fluid rotating within the separationchamber, the end cap 10 is formed with a projection 54 extend along thewall of the housing adjacent the inlet nozzle 42. The projection 54presents a helical surface 56 to the incoming fluid, forcing the fluidto flow in a helical path within the separation chamber 40. The generalform of the helical surface 56 is known in the art and described in GB2353236A.

It will be noted from FIG. 1 that the inlet bore 52 opens into theseparation chamber at a distance from the end cap 10. This provides avolume of the separation chamber between the inlet bore 52 and the endcap 10. This volume is of use when processing a fluid stream containinggas and permits the formation of a gas cap in the separation chamber 40during operation, as will be described hereinafter.

The separation chamber 40 comprises two outlets for fluid. First, fluidmay leave the separation chamber through the choke element 22, as willbe described hereinafter. Fluid that does not exit through the chokeelement 22 is removed from the separation chamber 40 through the outlet46. In the region of the separation chamber 40 downstream of the chokeelement 22, that is between the choke element and the outlet 46, thechoke assembly 2 is provided with features to encourage further fluidphase separation. The choke assembly 2 is configured to reduce thecross-sectional area of flow available within the separation chamber asfluid approaches the outlet 46.

This may be achieved in a number of ways. Referring to FIG. 1, afrusto-conical flow guide 60 is disposed around the central mandrel 30in the region downstream of the choke element. The effect of the flowguide 60 is to reduce the cross-sectional area available for the flow offluids in the annular cavity between the mandrel 30 and the inner wallof the housing 4. A generally cylindrical skirt 62 extends in thedownstream direction from the downstream end of the flow guide 60. Aplurality of longitudinal conduits 64 extend through the flow guideadjacent the mandrel 30 and serve to connect the region of theseparation chamber 40 downstream of the flow guide with the upstreamregion.

As an alternative or in addition to the frusto-conical flow guide 60,the housing 4 may be provided with a conical or tapered form in theregion of the separation chamber 40 immediately downstream of the chokeelement 22.

Downstream of the flow guide 60 and adjacent the outlet 46, thecross-sectional area of the separation chamber 40 is further reduced bya tapered flow guide 66, extending upwards (as viewed in FIG. 1) withinthe housing 4 from the second cap 14. An alternative to the tapered flowguide 66 of FIG. 1 is shown in FIG. 5. The features of the chokeassembly of FIG. 5 common to those of FIG. 1 are indicated using thesame reference numerals. The choke assembly of FIG. 5 has the mandrel 30formed with a tapered section 68 in the region adjacent the end cap 14and the outlet 46, such that the cross-sectional area of the separationchamber 40 reduces in the downstream direction along the tapered section68.

Downstream of the flow guide 60, the choke assembly 2 is provided with aplurality of vanes 70 extending longitudinally along the mandrel 30. Thevanes 70 act to inhibit the rotation of the fluid in the central zonewithin the adjacent region of the separation chamber. In this region, itis preferred to maintain some rotational components to the fluid flow,in order to maintain the fluid in an agitated state, for example toensure entrained solids are kept in suspension.

The flow of the fluid stream leaving the separation chamber 40 throughthe outlet 46 is controlled by a flow control assembly 72, ofconventional design.

In operation, the choke assembly of FIGS. 1 and 5 functions as follows:

The operation of the choke assembly will be described having referenceto a multiphase fluid stream comprising gas, oil, water and entrainedsolid particles, typical of a fluid stream produced from a subterraneanwell. In this fluid stream, the lightest fluid phase is the gas. Of theliquid phases, oil is the lighter phase and water is the heavier phase.The solid material, as it is entrained in the fluid, will behave as theheaviest fluid stream. It will be understood that this fluid stream ismerely exemplary of the fluid streams that can be processed using thechoke assembly of this invention and the scope of the present inventionis not to be limited to such fluid streams.

The fluid stream is introduced into the inlet end 6 of the separationchamber 40 through the inlet bore 52 of the inlet nozzle 42 from theinlet assembly 48. The action of the inlet assembly 48 has been toprecondition the fluid stream and initiate separation of the fluidphases. The fluid is introduced into the separation chamber 40 as shownin FIG. 3, such that the water and entrained solids are concentrated atthe inner wall of the housing 4, with oil and gas being introduced intothe separation chamber 40 at some distance from the wall. Thisorientation of the fluid phases matches that assumed by the variousphases within the separation chamber 40. The effect of the angle ofentry of the fluid and the helical fluid guide surface 56 is to causethe fluid to flow in a helical pattern, moving generally downstream inthe separation chamber 40, as represented in FIG. 4. The gas separatesfrom the liquid phases and forms a central gas core, stabilised by a gascap formed in the inlet end 6 of the separation chamber 40 above (asviewed in FIG. 1) the inlet 52 and adjacent the end cap 10. A gas/liquidinterface will form within the separation chamber 40, as represented inFIG. 4, with the liquid phases forming a lining around the wall of theseparation chamber 40, surrounding a gaseous core. The interface willhave the general ‘bowl’ shape shown in FIG. 4. Within the liquid phases,oil will collect in the region adjacent the gas/liquid interface, whilewater and entrained solids will concentrate and migrate to the radiallyouter region of the separation chamber 40 adjacent the inner wall of thehousing 4. The action of maintaining a gaseous core within the liquid‘bowl’, prevents liquids and entrained solids from collecting in thecentral region of the separation chamber. This in turn requires theliquids and entrained solids to collect in the radially outer region,adjacent the wall of the separation chamber, where they are subjected tohigher centrifugal forces, further enhancing separation of the phases.

The choke element 22 is located within the separation chamber 40 suchthat the gas/liquid interface intersects the choke cage, allowing thelighter fluid phases, in particular gas and oil, to leave the separationchamber through the choke element. Some water and solid particles may beentrained with the gas, oil and water stream and leave through the chokeelement. However, a portion of the water with a concentration of solidmaterial remains in the separation chamber 40. The fluid stream passingthrough the choke element 22 enters the bore 32 in the mandrel 30 andleaves the choke assembly 2 through the outlet 34 in the end cap 14.

The heavier fluid phases, in particular water and entrained solids passdownstream of the choke element 22 and pass the conical flow guide 60.The decreasing cross-sectional area within the separation chamber 40 asthe fluid flows downstream causes the density of solids to increase inthe fluid phase. The skirt 62 extending from the flow guide 60 providesa calm region immediately downstream of the flow guide 60. Lighterfluids, in particular gas and oil, entrained with the water and solidscollect beneath (as viewed in FIG. 1) the flow guide 60 and flow upwardsthrough the conduits 64 to pass through the choke element 22. Rotationof the water/solid mixture downstream of the flow guide 60 is damped bythe vanes 70, allowing the separation of lighter and heavier fluidphases and solid material to occur through a combined rotation andgravity separation regime. Water and solid material are withdrawn fromthe separation chamber 40 through the outlet 46, under the control ofthe flow control assembly 72.

Referring to FIG. 6, there is shown a further embodiment of a chokeassembly according to the present invention. The choke assembly of FIG.6 has the same general arrangement and configuration as that of FIGS. 1and 5 and the components and features common to both embodiments areindicated using the same reference numerals. The follow description willconcentrate on the features of the choke assembly of FIG. 6 that are notshared with the assemblies of FIGS. 1 and 5.

The choke assembly 2 of FIG. 6 comprises a cap assembly 110, in place ofthe end cap 10 at the inlet end 6 of the assembly. The cap assembly 110is secured to the housing 4 by means of bolts 112, in conventionalmanner. The cap assembly 110 comprises a central mandrel 18 extendingcoaxially into the housing 4 to support the choke element 22, ashereinbefore described. The cap assembly 110 comprises a gas/liquidseparation chamber 114 formed as a generally cylindrical cavity in thecap. The separation chamber 114 has a first outlet 116 for liquid, theopening of which extends from through the cap at the end of theseparation chamber 114 adjacent the housing 4. The first fluid outlet116 terminates in a conventional flanged coupling and is arranged withan opening that is tangential to the separation chamber 114, so as toallow the efficient removal of liquid from a rotating fluid stream inthe chamber. The separation chamber 114 has a second outlet 118extending from the end of the chamber distal to the housing 4 andterminating in a conventional flanged coupling. Again, the second outlet118 is arranged to have an opening that is tangential to the separationchamber 114. The cap assembly 110 comprises an end cap 120 mounted tothe end of the cap assembly and closing the separation chamber 114. Achoke actuator 122 of conventional design is mounted on the exterior ofthe end cap 120.

As described hereinbefore and shown in FIGS. 1 and 6, the mandrel 18 hasa central longitudinal bore 26 in which is housed a stem 28. In theembodiment shown in FIG. 6, the bore 26 extends through the end capassembly 110 and opens into the separation chamber 114. A furtherlongitudinal bore 126 extends through the end cap 120. A stem 128extends from the actuator 122 through the bore 126 in the end cap intothe separation chamber 114. A stem conduit 130 is connected to the freeend of the stem 128 within the separation chamber 114 and extendsthrough the bore 26 in the cap assembly into the choke element 22. Theend portion of the stem conduit 130 within the choke element 22 formsthe plug of the choke element. The stem conduit 130 is generallycylindrical and has a longitudinal bore 132 extending therethrough alongits entire length. The bore 132 in the stem conduit 130 is open at theend within the choke element 22 and provides an outlet for lighter fluidphases to leave the choke element in operation. The end portion of thestem conduit 130 extending into the separation chamber 114 in the capassembly 110 is provided with a plurality of tangential openings 134,through which fluid may leave the stem conduit 130 and enter theseparation chamber 114 in a rotating flow pattern.

The choke assembly 2 of FIG. 6 further comprises a cap assembly 140, inplace of the end cap 14 at the outlet end 6 of the assembly. The capassembly 140 is secured to the housing 4 by means of bolts 142, inconventional manner. The cap assembly 140 comprises a central mandrel 30of the type shown in FIG. 5 and extending coaxially into the housing 4to support the choke element 22, as hereinbefore described. The centralmandrel has a longitudinal bore 32 extending therethrough. The capassembly 140 further comprises a fluid collection chamber 144 formed bya tapered cavity extending longitudinally within the cap assembly 140from the bore 32 in the mandrel 30. The cavity is tapered so as to widenin the downstream direction towards a fluid outlet 146 terminating in aconventional flange coupling 148. The fluid outlet 146 is arranged tohave its opening extending tangentially from the collection chamber 144.An end cap 150 is mounted to the cap assembly 140 and closes thecollection chamber 144. A cylindrical vortex breaker 152 extends fromthe end cap 150 coaxially within the collection chamber, terminatingdownstream of the bore 32 in the mandrel 30. The vortex breaker 152serves to prevent a gas vortex extending from the choke element 22downstream into the collection chamber 144.

Operation of the choke assembly of FIG. 6 will now be described, usingas a reference example the aforementioned fluid stream.

Operation of the choke assembly upstream of the choke element 22 is asdescribed hereinbefore with reference to FIGS. 1 to 5. Passing throughthe choke cage is a fluid stream comprising a major portion of gas andthe lighter liquid phase, oil. Some minor amounts of heavier liquid,water, and solid material may be entrained in the lighter fluid stream.The choke element is arranged to generate a rotational fluid flow regimewithin the choke cage. The details of a choke element to achieve aredescribed below. The rotational flow regime causes the liquid phase tomigrate to the radially outer regions of the cavity within the chokeelement and the gas phase to collect in the central region. The gasflows from the choke element into the bore 132 in the stem conduit 130and flows through the bore, exiting through the tangential openings 134into the separation chamber 114. The arrangement of the tangentialopenings 134 causes the fluid in the separation chamber 114 to rotateand swirl. Liquid and heavier fluids, such as oil and minor quantitiesof water, entrained in the gas migrate to the radially outer region ofthe separation chamber 114 and pass to the outlet 116. Gas entering theseparation chamber 114 collects in the central region and exits throughthe outlet 118.

Heavier fluid phases, in particular oil and minor quantities of waterand entrained solids, leave the choke element and flow downstream alongthe bore 32 in the mandrel 30 to the collection chamber 144 in the capassembly 140. The tendency of the rotating gas stream to vortexdownstream into the collection chamber 144 is damped or inhibited by thevortex breaker 152. The liquid, still flowing in a rotating flow regime,leaves the collection chamber 144 through the outlet 146.

Referring to FIG. 7, there is shown a further embodiment of a chokeassembly according to the present invention. The choke assembly of FIG.7 is of the same general configuration as that shown in FIG. 6 anddescribed above. Features and components common to the assemblies ofFIGS. 6 and 7 are indicated using the same reference numerals.

The choke assembly of FIG. 7 differs from that of FIG. 6 by comprisingtwo fluid inlets for introducing fluid into the separation chamber 40. Afirst fluid inlet 202 is provided in the housing 4 and is disposedbetween the choke element 22 and the inlet end 6 of the housing. Thefirst inlet 202 terminates in a conventional flange coupling 204. Theinlet 202 is an angled, tangential inlet having the same arrangement andconfiguration as described above and shown in FIG. 3. An inlet assembly206 is connected to the inlet 202 and has the general configurationdescribed hereinbefore and shown in FIG. 2 a. The inlet assembly servesto precondition the incoming fluid stream, as described above. The firstfluid inlet is for a multiphase fluid stream, in particular a streamcomprising multiple liquid phases and optionally suspended or entrainedsolid material.

A second fluid inlet 210 is provided in the housing 4 and is disposedbetween the first fluid inlet 202 and the inlet end 6 of the housing.The second inlet 210 terminates in a conventional flange coupling 212.The inlet 210 is an angled, tangential inlet having the same arrangementand configuration as described above and shown in FIG. 3. An inletassembly 214 is connected to the inlet 210 and has the generalconfiguration described hereinbefore and shown in FIG. 2 a. The inletassembly serves to precondition the incoming fluid stream, as describedabove. The second fluid inlet is for a multiphase fluid stream, inparticular a stream comprising a major portion of gas with entrainedliquid droplets or condensate.

Overall, the operation of the assembly of FIG. 7 is as described abovewith reference to the choke assembly of FIG. 6. A multiphase fluidstream comprising a major portion of liquid phases is introduced via thefirst inlet 202 into the separation chamber and forms a helical flowregime below the projection 54, which presents a lower helical surfaceof a dual wall ramp 56 to the flow. A predominantly gas stream isintroduced into the separation chamber 40 through the second inlet 210directly into a gas cap formed within the separation chamber 40 upstreamof the first inlet. A projection 55 presents an upper helical surface 57of a dual wall ramp to the incoming fluid. The general flow patternwithin the separation chamber 40 is represented in FIG. 8. As shown, agas/liquid interface is established, with the liquid lining the outerwall of the separation chamber 40 and surrounding a gaseous core.Entrained liquid droplets in the gas stream entering through the secondinlet 210 are captured in the liquid phase. Similarly, gas present inthe liquid stream entering through the first inlet 202 is caused toleave the liquid phase and enter the gaseous core. The remainder of theoperation is as hereinbefore described.

The choke assembly of FIG. 7 is particularly useful in the processing oflarge volumes of both gaseous and liquid streams, where it is requiredto remove entrained liquid from the gas and to release gases from theliquid stream.

Referring to FIG. 9, there is shown a further embodiment of a chokeassembly according to the present invention. The choke assembly of FIG.9 has the same general configuration to that shown in FIG. 6, with theexception of the end cap assembly 140, which is replaced by an end capseparation assembly 200, the details of which are as follows:

The end cap separation assembly 200 of the assembly of FIG. 9 is mountedto the outlet end 8 of the housing bolts in conventional manner. The endcap assembly has a mandrel 30 extending coaxially into the housing 4,with all the features described above and shown in FIG. 6. The bore 32in the mandrel 30 opens into a liquid separation chamber 204. The liquidseparation chamber 204 is generally cylindrical in shape, having atapered section 206 adjacent the end of the bore 32 in the mandrel andwidening in the downstream direction. The generally cylindricalseparation chamber 204 is closed by an end cap 208, mounted to the endcap assembly 200 by bolts in conventional manner. The end cap 208 has aconical projection 210 extending into the separation chamber 204, toreduce the cross-sectional area of the separation chamber 204 in theregion immediately upstream of the end cap 208. A cylindrical conduit212 extends through a bore in the end cap 208 and coaxially into theseparation chamber 204. The cylindrical conduit 212 is provided with aplurality of tangential openings 214, through which fluid from theseparation chamber 204 can enter the conduit 212. The distal end of theconduit is closed and is capped by a vortex arrestor 216, extendingcoaxially within the separation chamber 204 towards the opening of theconduit 32 in the mandrel 30.

A first liquid outlet 218 is mounted to the exterior of the end cap 208and provides a flowpath for fluid leaving the assembly. A second fluidoutlet 220, terminating in a conventional flanged coupling, is providedin the end cap assembly 200 adjacent the end cap 210, for fluid leavingthe separation chamber 204.

In operation, the choke assembly of FIG. 9 performs as described abovewith reference to FIG. 6 upstream of the bore 32 in the mandrel 30extending from the end cap 140. In the choke assembly of FIG. 9, fluid,predominantly lighter liquid, in particular oil, with some entrainedheavier liquid, that is water, flows downstream from the bore 32 in themandrel 30 and enters the upstream end of the separation chamber 204 inthe end cap separation assembly 200. The tendency of gas from the chokeelement 22 to form a vortex down into the separation chamber 204 isprevented by the vortex breaker 216, resulting in a substantially liquidstream flowing through the separation chamber 204. The liquid stream isrotating, under the action of the choke element, as will be describedhereinafter. The action of the rotation is to cause the heavier liquidphase, water, to migrate to the outer wall of the separation chamber 204and for the lighter liquid phase, oil, to concentrate in the radiallycentral region. Oil gathered in this central region passes through thetangential openings 214 in the conduit 212 and flows downstream throughthe end cap 208 to the first outlet 218. The oil takes an upwardsflowpath over a level weir 219, before exiting through an outlet 217.Water, the heavier liquid phase, remains in the separation chamber 204and flows downstream to the second outlet 220 in the end cap assembly.

Referring to FIG. 10, there is shown an alternative embodiment of thechoke assembly of FIG. 9, having two fluid inlets, as shown in FIG. 7and described hereinbefore.

As noted above, a preferred choke element for use in the chokeassemblies of the present invention is one in which the fluid passingthrough the choke element is caused to flow in a rotational flow regimewithin the choke element. As noted above, this rotational flow of fluidallows for various separation stages to take place, enhancing theprocessing of multiphase fluid streams using the choke assembly. Apreferred arrangement of choke element is shown in FIG. 11 and will nowbe described.

Referring to FIG. 11, there is shown a choke element, generallyindicated as 300, in place in the choke assembly of FIG. 1. It will beunderstood that the choke assembly 300 is not limited in its use to thechoke assembly of FIG. 1 and can equally well be applied in the chokeassemblies of other embodiments of the present invention, as well aschoke assemblies falling outside the scope of the invention. The lefthand side of FIG. 11 shows the choke element in the fully closedposition, while the right hand side of the figure shows the chokeelement in the fully open position.

The choke element 300 is of the plug and cage variety and comprises agenerally cylindrical cage 302 having an inner cage portion 304 and anouter cage portion 306 arranged concentrically. The outer cage portion306 is supported between the mandrel 18 extending from the end cap atthe inlet end within the recess 20 in the end of the mandrel 18 and themandrel 30 extending from the end cap at the outlet end. The chokeelement 300 further comprises a generally cylindrical plug 308 disposedwithin the cage 302. The plug 308 is connected in conventional manner tothe end of the stem 28 extending from the actuator mounted to the endcap at the inlet end of the housing. In this manner, the plug 308 may bemoved longitudinally within the cage by a reciprocating motion of thestem 28 under the action of the actuator.

As noted, the choke element 300 shown in FIG. 11 has the plug disposedwithin the cage. It will be understood that the present inventionembraces a similar choke element, but in which the plug is disposedoutside the cage. To achieve a ‘shut-off’ that is complete closure ofthe choke assembly, the plug has a seat 309 which seals on a cageshoulder 307.

The cage 302 is provided with a plurality of openings 310, through whichfluid may flow from the separation chamber 40 into the central cavitywithin the choke element 300. The openings 310 are formed to extendtangentially through the cage 302. Referring to FIG. 12, there is showna cross-sectional view through a portion of the choke cage 302 showingthe form of an opening 310. Each opening 310 extends through both theouter cage portion 304 and the inner cage portion 306. The openings aregenerally rectangular in cross-section. The portion of each openingextending through the inner cage portion 306 has a constantcross-sectional area throughout its length. The portion of each openingextending through the outer cage portion 304 is tapered, with thecross-sectional area of the opening at the outer surface of the outercage portion being widest, the opening tapering in a radially inwardsdirection such that the cross-sectional area reduces to that of theopening portion in the inner cage portion. As shown in FIG. 12, thetaper of the opening portion in the outer cage portion 304 is notsymmetrical, but rather is offset in the reverse direction of fluid flowaround the outside of the cage, as indicated by arrows A.

The inner cage portion 306 is formed from tungsten, with the exposedopenings acting as the fluid throttling means. The outer cage portion304 is formed from stainless steel. Tungsten is a very hard material,resistant to erosion. However, tungsten is a brittle material and iseasily fractured upon impact by solid material. Accordingly, the outercage portion 306, being of stainless steel, while less resistant toerosion, is less brittle than the tungsten inner portion and better ableto resist fracturing under impact.

The fluid flow patterns in and around the cage 302 are shown in FIG. 12.Fluid in the separation chamber 40 is flowing in a rotating patternaround the exterior of the cage 302, as indicated by arrows A. As thefluid passes the outer end of an opening 310, a portion of the fluid isdirected into the opening, as indicated by arrow B. The offset in thetaper in the outer portion of the opening reduces the shear experiencedby the fluid as it enters the opening 310. As the fluid passes throughthe opening, the reduction in cross-sectional area accelerates the fluidvelocity, such that upon entering the inner cavity of the choke element,the fluid is travelling at a high velocity. The angle of the opening 310introduces the high velocity fluid into the inner cavity tangentially,as indicated by arrow C. As a result of the tangential entry, the fluidwithin the inner cavity is caused to rotate, as shown by arrows D. Thus,the incoming fluid indicated by arrow C is caused to change direction,as shown in FIG. 12, and follow a flow path close to the inner surfaceof the inner cage portion 306. Overall, this pattern of flow reduces thedegree of shear to which the fluid is subjected, which in turn has theeffect of maintaining phase separation that exists in the fluid prior toentering the cage. In addition, as will be appreciated, any solidparticles entrained in the fluid are caused to take a circular pathwithin the choke cage 302, rather than impact equal and opposite fluidjets within the cage, as with choke assemblies of conventional design.This significantly reduces fluid shear, erosion of the cage and failurerates of the choke element.

As shown in FIG. 11, the openings in the choke cage 302 are arranged inbands. Cross-sectional views through the six downstream bands of thechoke cage 302 are shown in FIGS. 13 a to 13 f. The most downstream bandis shown in FIG. 13 a and has two openings 310 arranged at 180° to eachother on opposing sides of the cage 302. Similarly, the adjacentupstream band, shown in FIG. 13 b, has two openings arranged at 180° toeach other on opposing sides of the cage, but offset by 90° to theopenings in the band shown in FIG. 13 a. The adjacent upstream band isshown in FIG. 13 c and has four openings arranged at 90° to each otheraround the cage. The band immediately upstream is shown in FIG. 13 d,which also comprises four openings with a 90° spacing, again offset fromthe four openings of the adjacent downstream band by 45°. The upstreampair of bands is shown in FIGS. 13 e and 13 f, each band having eightopenings.

Movement of the plug 308 longitudinally within the cage 302 allows thebands of openings to be covered and uncovered, depending upon theposition of the plug, thereby allowing the flow of fluid through thechoke element to be controlled. With the plug in the position shown inthe left hand side of FIG. 11, all bands of openings are covered and thechoke element is closed to fluid flow. Movement of the plug 308 from theclosed position first uncovers the band shown in FIG. 13 a and then,successively, the bands of FIGS. 13 b to 13 f, until the fully openposition shown in the right hand side of FIG. 11 is reached. It will beappreciated that movement of the plug from the closed position to theopen position opens bands having successively a greater number ofopenings and, hence, providing a successively increasing cross-sectionalarea available for fluid flow.

It has been found that the arrangement of the tangential openings 310into bands, as described hereinbefore, induces a particular flow regime,represented in FIGS. 14 a and 14 b. As described hereinbefore and shownin FIG. 12, the fluid entering the cavity within the cage element iscaused to flow in a circular path adjacent the inner surface of theinner cage portion 306, that is form a rotating band of pressurisedfluid in the radially outer region of the cavity. The pressurisedcentrifugal band of fluid 400 generated by the downstream band ofopenings is shown in FIGS. 14 a and 14 b. The general direction of fluidflow within the choke element is downstream towards the bore 32 in themandrel 30 (that is downwards, as shown in FIGS. 14 a and 14 b).However, the effect of the pressurised centrifugal band 400 is to reducethe effective cross-sectional area available for the flow of fluidentering the choke element upstream of the band 400. In other words, thepressurised centrifugal band of fluid acts as a hydraulic choke to theflow of fluid downstream. A similar pressurised centrifugal band ofrotating fluid 402 will be formed by the adjacent upstream band ofopenings, which acts to further reduce the effective cross-sectionalarea available for the downstream flow of fluid.

As shown in FIG. 14 a, the choke element is in the partially openposition, with the plug 308 in an intermediate position between thefully open and fully closed positions and two bands of openingsuncovered for fluid flow. The effect of the hydraulic choke actionwithin the choke cavity is represented. In FIG. 14 b, the plug 308 is inthe fully open position, with all bands of openings uncovered. Thehydraulic choke action of successive fluid bands is shown. The fluidentering the cavity through a given band of openings must flow in thegeneral downstream direction. To do this, the fluid is caused to flowinwards, past the pressurised centrifugal band of fluid established bythe fluid entering the adjacent downstream band of openings. As shown inFIG. 14 b, the cumulative effect of the pressurised centrifugal bands offluid results in the fluid entering the choke cavity through the mostupstream bands of openings having a very limited cross-sectional areaavailable for flow past the downstream fluids bands.

The hydraulic choke action of the pressurised centrifugal fluid bandsacts in addition to the physical choking action provided by the flow offluid through the openings in the choke cage. Thus, the choke employsthree different mechanisms for choking the flow of fluid: the mechanicalchoking effect provided by the fluid flowing through restricted orifices(as with conventional choke designs); the hydraulic choking effectcaused by the rotating bands of fluid within the choke cage providingresistance to the entry of fluid into the cage; and the hydraulicchoking effect of the bands of fluid creating hydraulic orifices (asshown in FIGS. 14 a and 14 b) providing resistance to the general fluidflow within the cage. The hydraulic choke action can be employed tocontrol the flow of fluid downstream of the choke element, but has theadvantage of subjecting the fluid to significantly less shear than thecomponents of the choke element, such as would be experienced in aconventional plug and cage choke assembly.

What is claimed is:
 1. A method of controlling and separating the flowof multiphase fluid streams, the method comprising: introducing a firstfluid stream rich in lighter fluid phases into a separation zone;introducing a second fluid stream rich in heavier fluid phases into aseparation zone to form a combined fluid stream in which the first fluidstream and the second fluid stream are in contact; allowing separationof the fluid phases in the combined fluid stream to occur in theseparation zone; causing lighter fluid phases separated in theseparation zone to flow through a choke element to a first outlet;controlling the flow of fluid phases through the choke element byadjusting the choke element; and causing the remaining fluid phases toflow to a second outlet.
 2. The method according to claim 1, wherein thefirst fluid stream is introduced into the separation zone upstream ofthe second fluid stream.
 3. The method according to claim 2, whereineither one or both of the first and second streams is preconditionedprior to being introduced into the separation zone.
 4. The methodaccording to claim 3, wherein the preconditioning comprises initiatingseparation of the phases in the fluid stream.
 5. The method according toclaim 1, further comprising subjecting the fluid passing through thechoke element to further separation.
 6. A choke assembly comprising: achoke element for controlling the flow of fluid therethrough; a firstinlet for a lighter fluid stream; a second inlet for a heavier fluidstream; and at least one outlet for fluid.
 7. A choke assemblycomprising: a choke element comprising a moveable choke component; astem connected to the moveable choke component; wherein the stem has abore therethrough to provide an outlet for fluid from within the chokeelement; and wherein the choke element comprises a cage having aplurality of openings therein and a plug, the plug being moveablelongitudinally with respect to the cage.
 8. The choke assembly accordingto claim 7, wherein the plug is disposed inside the cage.
 9. The chokeassembly according to claim 7, wherein the choke element is arranged toinduce a rotational flow pattern in fluid passing through the chokeelement.
 10. The choke assembly according to claim 7, further comprisingmeans for subjecting fluid leaving through the outlet to furtherseparation.
 11. The choke assembly according to claim 10, wherein thesaid means comprises a separation chamber.
 12. A method for controllingand separating a multiphase fluid stream, the method comprising: passingthe fluid stream through a choke element and controlling the flow offluid using the choke element; introducing the fluid stream into aseparation zone and causing phases of the fluid stream to separate; andremoving a lighter fluid phase from the separation zone through anactuation stem extending from the choke element.
 13. A choke assemblycomprising a choke element comprising: a cage having a plurality ofopenings therethrough for the passage of fluid; a plug moveable withrespect to the cage to open and close the openings in the cage; andwherein the openings in the cage extend tangentially to the cage, eachopening comprising an outer portion extending from the outer surface ofthe cage having a first cross-sectional area and an inner portionextending from the inner surface of the cage having a secondcross-sectional area, the first cross-sectional area being greater thanthe second cross-sectional area.
 14. The choke assembly according toclaim 13, wherein the change in the cross-sectional area of each openingis gradual or continuous through the cage.
 15. The choke assemblyaccording to claim 14, wherein at least a portion of each opening iscurved and/or tapered.
 16. The choke assembly according to claim 15,wherein the taper is asymmetrical, being offset in the reverse directionof flow of fluid around the exterior of the cage.
 17. The choke assemblyaccording to claim 13, wherein the cage comprises an inner cage elementand an outer cage element arranged concentrically around the inner cageelement.
 18. The choke assembly according to claim 17, wherein theportion of each opening extending through the outer cage element reducesin cross-sectional area in the inward direction and the portion of eachopening extending through the inner cage element has a constantcross-sectional area.
 19. The choke assembly according to claim 13,wherein the ratio of the cross-sectional area of each opening at itsinner end to the cross-sectional area at its outer end is from 1:1.5 to1:5.
 20. A choke assembly comprising a choke element comprising: a cagehaving a plurality of openings therethrough for the passage of fluid; aplug moveable with respect to the cage to open and close the openings inthe cage; wherein the openings in the cage extend tangentially to thecage, the cage comprising openings arranged in a plurality of bandsextending circumferentially around the cage, the cross-sectional area ofthe openings of the bands increasing in the upstream direction of thecage.
 21. The choke assembly according to claim 20, wherein all theopenings have the same cross-sectional area, the increase incross-sectional area from band to band provided by an increasing numberof openings.
 22. The choke assembly according to claim 20, wherein thebands of openings are in groups, each group comprising a plurality ofbands with the bands in each group all having the same number ofopenings.
 23. The choke assembly according to claim 22, wherein eachgroup comprises two bands of openings.
 24. The choke assembly accordingto claim 20, wherein the openings in a given band are offsetcircumferentially from the openings in an adjacent band.
 25. The chokeassembly according to claim 20, wherein the plug is disposed within thecage.
 26. A method of controlling the flow of a fluid, the methodcomprising: introducing the fluid into a flow control zone having ageneral downstream direction in which the fluid is required to flow, thefluid being introduced into the flow control zone through a plurality ofopenings; wherein fluid introduced through a first group of openings,downstream of a second group of openings, establishes a generallytoroidal flow pattern within the flow control zone, whereby theeffective cross-sectional area available for the flow of fluidintroduced through the second group of openings in the downstreamdirection is reduced.
 27. The method according to claim 26, wherein thefirst group comprises a plurality of openings.
 28. The method accordingto claim 26, wherein fluid introduced through the second group ofopenings is caused to form a second generally toroidal flow patterndisposed upstream of the first group of openings.
 29. The methodaccording to claim 28, further comprising introducing fluid through athird group of openings to form a third generally toroidal flow patternupstream of the second.
 30. A choke installation comprising two or moreof a choke assembly as claimed in claim 6, 7, 13, or
 20. 31. A wellheadinstallation comprising a choke assembly as claimed in any of claim 6,7, 13, or
 20. 32. The wellhead installation according to claim 31,wherein the wellhead is a subsea wellhead.